A man in his seventies visited our hospital due to positive a fecal occult blood test. Colonoscopy showed a submucosal tumor in the rectum covered with reddish and irregular mucosa. Pathological examination of the biopsy specimen revealed well differentiated adenocarcinoma. The tumor was resected, and pathological examination showed carcinoid and adenocarcinoma components coexisting and mixed with each other. Because a previous colonoscopy had shown a submucosal tumor with a smooth surface, this case was diagnosed as a collision tumor of adenocarcinoma which developed on a carcinoid.